(1) Field of the Invention
The present invention relates to telephone trunk circuits and more particularly to an AC supervisory signal detector for use in a private automatic branch exchange to detect AC supervisory signals from a central office.
(2) Description of the Prior Art
The current state of the art in AC supervisory signal detectors is disclosed in U.S. Pat. No. 4,133,982 issued on Jan. 9, 1979 to D. Q. Lee, et al. However, this circuit suffers from a quick response time for detection of an AC supervisory signals and delayed recognition of removal of such signals. This quick response time, typically 20 milliseconds, increases the probability of erroneous detection of noise as a valid AC supervisory signal. The error represented by this false detection of an AC supervisory signal is compounded by the long response time, typically several hundred milliseconds, of this circuit to termination of a detected signal.
Accordingly, it is the object of the present invention to provide an AC supervisory signal detector which discriminates against noise signals and responds only to valid AC supervisory signals.